The present invention relates to a new and improved method for the recalibration or reforming of a worn, conical, especially curved tubular mold for use in continuous casting of strands.
Generally speaking, the method of the invention contemplates introducing a calibration arbor or mandril into the hollow compartment of the mold, applying an explosive charge to the outer surface of the tubular mold, detonating the explosive charge and through such detonation deforming or reforming the hollow compartment or cavity of the tubular mold to the dimensions of the calibration arbor or mandril.
During the continuous casting of metals, especially steel, there are used throughpass molds having straight or curved shapes. Generally, the walls of such molds, forming the hollow mold compartment or cavity are fabricated of copper or copper alloys. When continuously casting smaller strand sectional shapes, for instance billets and smaller bloom sections, as a general rule, these molds are formed of tubular elements or tubes. In order to compensate the shrinkage, frequently pronounced for many steel qualities, of the strand solidifying at its surface within the mold, in order to afford adequate strand cooling, it is conventional practice to provide the major portion of the tubular molds with a taper which converges towards the outfeed side of the strand. The continuous casting molds are exposed to wear, particularly at the region of the molten bath level within the mold and, especially, when the continuous casting mold is used at casting installations working with bath level regulation systems. This wear of the mold results in damage of the mold surface, such as typified for instance by large surface pitting and frequently deeply penetrating fissures, especially in the case of tubular molds at which there has been accomplished a high number of pours.
Moreover, unavoidable thermal stresses lead to a contraction of the tubular mold at the region of the molten bath level, and to an enlargement, which is intensified by abrasion or wear, at the strand withdrawal end of the continuous casting mold, thus resulting in undesired, incorrect taper.
The fabrication of a drawn, conical tubular mold with the requisite smallest possible tolerances, requires a great expenditure in equipment, and therefore, is associated with appreciable costs, especially in the case of tubular molds having a curved, hollow mold compartment. In order to increase the service life of such expensive tubular molds by repairing or reworking the same, there have been developed special techniques for recalibrating such molds. Tubular molds having different tapers could not be recalibrated.
At the present time, spent molds, exhibiting practically unchanged taper and essentially only insignificant damage or flaws, such as, for instance, surface fissures at the inner wall of the tubular mold, following the formation of a mold fissure are subjected to a mechanical machining operation, such as, for instance grinding, planing and so forth of the mold inner surface in order to rework the mold so that it can again be reused for casting purposes. Yet, such procedures are automatically associated with an enlargement in the format or shape of the mold, frequently amounting to several millimeters. In most instances, such enlargement of the dimensions of the mold cannot be tolerated because of the thereafter performed processing of the cast strand material, for instance at a rolling mill, owing to the there prevailing pass gauge.
These drawbacks have been partially overcome through the use of a technique known to the art from German Pat. No. 2,533,528, but up to now not very frequently employed. With this prior art technique it is possible to produce conical, but also different conical, bent or curved molds by deforming a blank over a die by means of an explosive charge, and additionally with the same technique to reduce in size a spent tubular mold which has been enlarged by wear back to its original mold dimensions. With this procedure, the walls of the hollow mold compartment, as a rule, again should have imparted thereto the characteristics of a new casting mold.
In practice, however it has been found that the deeply penetrating fissures which are formed due to the previously mentioned bath level regulation, no longer can be eliminated by the described recalibration technique. During the explosive deformation there results an overlapping of the walls of the fissures, so that there arise unacceptable surface defects or flows, even if there has been accomplished a subsequent chrome plating of the mold walls. Hence, it is not possible to reuse such processed tubular molds because of the poor surface quality of the cast strand which will be formed in such reprocessed molds. In particular, upon renewed use of such repaired tubular mold, back in its original casting position, the only partially closed wall fissures again are located at the region of the greatest thermal stresses, tend to again open-up and liquid steel could penetrate into the open fissures, which, in turn, could lead to metal break-out and furthermore, to the formation of holes in the mold wall associated with dangerous escape of the cooling water. On the other hand, deep grinding of the fissures, prior to the explosive deformation work, which would be necessary in order to avoid such overlapping of the walls of the fissures, would result in an impermissible weakening of the mold walls at the corresponding regions of the tubular mold which is thus machined. These continuous casting molds no longer can be employed in casting operations, and therefore, only have scrap value with the present state of this technology. Since it is becoming more and more commonplace to use bath level-regulation devices, it should be apparent that the number of such type damaged tubular molds is ever increasing. Hence, the advantage of the exact regulation of the molten bath level is adversely offset by increased mold rejects.